1 /*
2 * Copyright (c) 1997, 2021, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #ifndef SHARE_OOPS_MARKWORD_HPP
26 #define SHARE_OOPS_MARKWORD_HPP
27
28 #include "metaprogramming/integralConstant.hpp"
29 #include "metaprogramming/primitiveConversions.hpp"
30 #include "oops/compressedKlass.hpp"
31 #include "runtime/globals.hpp"
32
33 // The markWord describes the header of an object.
34 //
35 // Bit-format of an object header (most significant first, big endian layout below):
36 //
37 // 32 bits:
38 // --------
39 // hash:25 ------------>| age:4 unused_gap:1 lock:2 (normal object)
40 //
41 // 64 bits:
42 // --------
43 // nklass:32 hash:25 -->| unused_gap:1 age:4 unused_gap:1 lock:2 (normal object)
44 //
45 // - hash contains the identity hash value: largest value is
46 // 31 bits, see os::random(). Also, 64-bit vm's require
47 // a hash value no bigger than 32 bits because they will not
48 // properly generate a mask larger than that: see library_call.cpp
49 //
50 // - the two lock bits are used to describe three states: locked/unlocked and monitor.
51 //
52 // [ptr | 00] locked ptr points to real header on stack
53 // [header | 01] unlocked regular object header
54 // [ptr | 10] monitor inflated lock (header is wapped out)
55 // [ptr | 11] marked used to mark an object
56 // [0 ............ 0| 00] inflating inflation in progress
57 //
58 // We assume that stack/thread pointers have the lowest two bits cleared.
59 //
60 // - INFLATING() is a distinguished markword value of all zeros that is
61 // used when inflating an existing stack-lock into an ObjectMonitor.
62 // See below for is_being_inflated() and INFLATING().
63
64 class BasicLock;
65 class ObjectMonitor;
66 class JavaThread;
67 class outputStream;
68
69 class markWord {
70 private:
71 uintptr_t _value;
72
73 public:
74 explicit markWord(uintptr_t value) : _value(value) {}
75
76 markWord() = default; // Doesn't initialize _value.
77
78 // It is critical for performance that this class be trivially
79 // destructable, copyable, and assignable.
80 ~markWord() = default;
81 markWord(const markWord&) = default;
82 markWord& operator=(const markWord&) = default;
83
84 static markWord from_pointer(void* ptr) {
85 return markWord((uintptr_t)ptr);
86 }
87 void* to_pointer() const {
88 return (void*)_value;
89 }
90
91 bool operator==(const markWord& other) const {
92 return _value == other._value;
93 }
94 bool operator!=(const markWord& other) const {
95 return !operator==(other);
96 }
97
98 // Conversion
99 uintptr_t value() const { return _value; }
100
101 // Constants
102 static const int age_bits = 4;
103 static const int lock_bits = 2;
104 static const int self_forwarded_bits = 1;
105 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - self_forwarded_bits;
106 static const int hash_bits = max_hash_bits > 25 ? 25 : max_hash_bits;
107 #ifdef _LP64
108 static const int klass_bits = 32;
109 #endif
110
111 static const int lock_shift = 0;
112 static const int self_forwarded_shift = lock_shift + lock_bits;
113 static const int age_shift = self_forwarded_shift + self_forwarded_bits;
114 static const int hash_shift = age_shift + age_bits;
115 #ifdef _LP64
116 static const int klass_shift = hash_shift + hash_bits;
117 #endif
118
119 static const uintptr_t lock_mask = right_n_bits(lock_bits);
120 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift;
121 static const uintptr_t self_forwarded_mask = right_n_bits(self_forwarded_bits);
122 static const uintptr_t self_forwarded_mask_in_place = self_forwarded_mask << self_forwarded_shift;
123 static const uintptr_t age_mask = right_n_bits(age_bits);
124 static const uintptr_t age_mask_in_place = age_mask << age_shift;
125 static const uintptr_t hash_mask = right_n_bits(hash_bits);
126 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift;
127
128 #ifdef _LP64
129 static const uintptr_t klass_mask = right_n_bits(klass_bits);
130 static const uintptr_t klass_mask_in_place = klass_mask << klass_shift;
131 #endif
132
133 static const uintptr_t locked_value = 0;
134 static const uintptr_t unlocked_value = 1;
135 static const uintptr_t monitor_value = 2;
136 static const uintptr_t marked_value = 3;
137
138 static const uintptr_t no_hash = 0 ; // no hash value assigned
139 static const uintptr_t no_hash_in_place = (address_word)no_hash << hash_shift;
140 static const uintptr_t no_lock_in_place = unlocked_value;
141
142 static const uint max_age = age_mask;
143
144 // Creates a markWord with all bits set to zero.
145 static markWord zero() { return markWord(uintptr_t(0)); }
146
147 // lock accessors (note that these assume lock_shift == 0)
148 bool is_locked() const {
149 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
150 }
151 bool is_unlocked() const {
152 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
153 }
154 bool is_marked() const {
155 return (mask_bits(value(), lock_mask_in_place) == marked_value);
156 }
157 bool is_neutral() const {
158 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
159 }
160
161 // Special temporary state of the markWord while being inflated.
162 // Code that looks at mark outside a lock need to take this into account.
163 bool is_being_inflated() const { return (value() == 0); }
164
165 // Distinguished markword value - used when inflating over
166 // an existing stack-lock. 0 indicates the markword is "BUSY".
167 // Lockword mutators that use a LD...CAS idiom should always
168 // check for and avoid overwriting a 0 value installed by some
169 // other thread. (They should spin or block instead. The 0 value
170 // is transient and *should* be short-lived).
171 static markWord INFLATING() { return zero(); } // inflate-in-progress
172
173 // Should this header be preserved during GC?
174 bool must_be_preserved(const oopDesc* obj) const {
175 return (!is_unlocked() || !has_no_hash());
176 }
177
178 // WARNING: The following routines are used EXCLUSIVELY by
179 // synchronization functions. They are not really gc safe.
180 // They must get updated if markWord layout get changed.
181 markWord set_unlocked() const {
182 return markWord(value() | unlocked_value);
183 }
184 bool has_locker() const {
185 return ((value() & lock_mask_in_place) == locked_value);
186 }
187 BasicLock* locker() const {
188 assert(has_locker(), "check");
189 return (BasicLock*) value();
190 }
191 bool has_monitor() const {
192 return ((value() & lock_mask_in_place) == monitor_value);
193 }
194 ObjectMonitor* monitor() const {
195 assert(has_monitor(), "check");
196 // Use xor instead of &~ to provide one extra tag-bit check.
197 return (ObjectMonitor*) (value() ^ monitor_value);
198 }
199 bool has_displaced_mark_helper() const {
200 return ((value() & unlocked_value) == 0);
201 }
202 markWord displaced_mark_helper() const;
203 void set_displaced_mark_helper(markWord m) const;
204 markWord copy_set_hash(intptr_t hash) const {
205 uintptr_t tmp = value() & (~hash_mask_in_place);
206 tmp |= ((hash & hash_mask) << hash_shift);
207 return markWord(tmp);
208 }
209 // it is only used to be stored into BasicLock as the
210 // indicator that the lock is using heavyweight monitor
211 static markWord unused_mark() {
212 return markWord(marked_value);
213 }
214 // the following two functions create the markWord to be
215 // stored into object header, it encodes monitor info
216 static markWord encode(BasicLock* lock) {
217 return from_pointer(lock);
218 }
219 static markWord encode(ObjectMonitor* monitor) {
220 uintptr_t tmp = (uintptr_t) monitor;
221 return markWord(tmp | monitor_value);
222 }
223
224 // used to encode pointers during GC
225 markWord clear_lock_bits() { return markWord(value() & ~lock_mask_in_place); }
226
227 // age operations
228 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
229 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
230
231 uint age() const { return mask_bits(value() >> age_shift, age_mask); }
232 markWord set_age(uint v) const {
233 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
234 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
235 }
236 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
237
238 // hash operations
239 intptr_t hash() const {
240 return mask_bits(value() >> hash_shift, hash_mask);
241 }
242
243 bool has_no_hash() const {
244 return hash() == no_hash;
245 }
246
247 #ifdef _LP64
248 inline Klass* klass() const;
249 inline Klass* klass_or_null() const;
250 inline Klass* safe_klass() const;
251 inline markWord set_klass(const Klass* klass) const;
252 inline narrowKlass narrow_klass() const;
253 inline markWord set_narrow_klass(const narrowKlass klass) const;
254 #endif
255
256 // Prototype mark for initialization
257 static markWord prototype() {
258 return markWord( no_hash_in_place | no_lock_in_place );
259 }
260
261 // Debugging
262 void print_on(outputStream* st, bool print_monitor_info = true) const;
263
264 // Prepare address of oop for placement into mark
265 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
266
267 // Recover address of oop from encoded form used in mark
268 inline void* decode_pointer() { return (void*)clear_lock_bits().value(); }
269
270 inline bool self_forwarded() const {
271 return mask_bits(value(), self_forwarded_mask_in_place) != 0;
272 }
273
274 inline markWord set_self_forwarded() const {
275 return markWord(value() | self_forwarded_mask_in_place | marked_value);
276 }
277 };
278
279 // Support atomic operations.
280 template<>
281 struct PrimitiveConversions::Translate<markWord> : public TrueType {
282 typedef markWord Value;
283 typedef uintptr_t Decayed;
284
285 static Decayed decay(const Value& x) { return x.value(); }
286 static Value recover(Decayed x) { return Value(x); }
287 };
288
289 #endif // SHARE_OOPS_MARKWORD_HPP